A body/B body housngs length?

You were told wrong. No B-body housing is narrower than an A-body housing like Kim said. No idea on what your offset needs to be as any time you modify something it changes. I'd say build the rearend to work with your wheels since you don't want to buy new ones. If the wheels were designed for a stock size A-body rear use a stock size A-body rear so the wheels will fit.